1. Field of the Invention
This invention concerns size reduction machines, and in particular to a screen holder for use with a size reduction machine that positions and compressively locks a screen associated with the size reduction machine in place. This invention also relates to a mechanism for setting the gap between the impeller and the screen of a size reduction machine. This invention further relates to size reduction machines that can be easily disassembled for cleaning.
2. Description of the Art
Maintaining the gap between the impeller and the screen of a size reduction machine is important in controlling product particle size. Therefore, it is imperative that the gap dividing the size reduction machine impeller from the size reduction machine screen is held constant during size reduction machine use. Furthermore, since a variety of screen sizes and impeller designs can be used within a single size reduction machine to produce products having a wide range of particle sizes, it additionally becomes important to be able to consistently adjust the gap between the size reduction machine screen and the size reduction machine impeller to control product particle size. Being able to adjust the impeller/screen gap is also important to maintain geometric screen uniformity because any non-uniformity such as warpage can detrimentally effect product particle size and/or particle size distribution.
Some size reduction machines of the prior art use frusto-conical shaped screens located in a channel between an input and an output. Such a size reduction machine is disclosed, for example, in U.S. Pat. No. 4,759,507, which describes using various screen openings of varying size and shape and using various impeller types to control particle size. According to the '507 patent, once a screen and impeller have been selected, the operation and efficiency of the machine depends upon the gap between the impeller and the interior wall surface of the screen. With the '507 patent device, different wall thickness screens are compensated for by inserting or removing spacers on the impeller shaft in order to move the impeller relative to the interior wall surface of the screen. Since the wall of the screen is tapered relative to the impeller, the actual adjustment of the gap is less than the thickness of the spacer and depends upon the angle of the screen relative to the horizontal. Where the tapered wall of the screen has an angle of sixty degrees relative to the horizontal, the gap is adjusted by one half the thickness of the spacer.
The use of spacers to control the screen/impeller gap creates difficulties. The process of installing a spacer and repeatedly removing and replacing incremental spacers is time consuming. Further, since the spacers must be incrementally sized and machined, the cost of producing such spacers is relatively high. Spacers are also easily lost during cleaning which can lead to re-assembly of the size reduction machine with an improper gap setting and decreased performance.
Adjustable size reduction machines without spacers are known in the art. For example, U.S. Pat. Nos. 4,773,559, 4,759,507, and 4,768,722 disclose machines in which the gap between the impeller and the screen is determined by the thickness of the screen flange or in which the gap is set by indexing the axial position of the impeller shaft when the machine is not in operation.
U.S. Pat. No. 5,282,579 discloses a size reduction machine with an adjustable impeller shaft. The impeller shaft is constructed in two parts that are united by a spacer device that operates much like a caliper to adjust the impeller shaft length, and thereby the gap between the impeller and the screen. One problem with the '579 patent device is that the gap cannot be adjusted after the size reduction machine is assembled.
U.S. Pat. No. 4,605,173 discloses a size reduction machine with an adjustable stop for limiting the maximum travel of the impeller into the frusto-conical screen. U.S. Pat. No. 5,505,392 discloses a size reduction machine having an adjustable length rotary drive coupling. The coupling includes two rotary shafts, one of which has at least one tooth and the second of which includes varying depth abutment surfaces. The union of a tooth with an abutment surface sets the gap between the impeller and the frusto-conical screen of the size reduction machine.
Despite the advances made in size reduction machine design, there remains a need for size reduction machines with improved mechanisms for setting the impeller to screen gap. Specifically, there remains a need to be able to provide simple, positive and accurate incremental adjustment of the screen to impeller gap without disassembling the size reduction machine. In addition there is a need for devices that maintain geometry of a frusto-conical screen and its concentric alignment with respect to the impeller shaft. There is further a need for a size reduction machine that is easily disassembled for cleaning.